Efficient mass exchange between liquid and gaseous phases is the key to successfully performing a range of different chemical engineering operations including distillation. Distillation is performed industrially to separate single components from fluid mixtures comprising two or more components. In a distillation column contact between the liquid phase and gaseous phase is carried out on vertically spaced horizontal trays. Liquid flows down the column from tray to tray and comes into contact with gas ascending the column through passages formed in the trays. Bubble cap trays were the first kind of trays to be used in large-scale distillation operations. These trays have a multiplicity of tubular risers which extend upwardly from the surface of the tray and are covered by caps with serrated edges or slots so that the gas can pass through the caps into the liquid on the surface of the trays. Bubble cap trays provide intimate contact between gas and liquid but are expensive to fabricate and are in operation subject to high gas pressure drops thereby increasing the work that has to be done to separate the fluid mixture. This latter disadvantage becomes particularly significant when the fluid mixture comprises two or more components which have similar volatilities to one another. Air is an example of such a mixture.
Accordingly, distillation columns used in, for example, modern air separation plants have sieve trays (or plates) instead of bubble cap trays. The construction of a sieve tray is simple. It is generally circular in shape and has a multiplicity smooth-edged circular apertures each having a vertical axis. In comparison with bubble cap trays, sieve trays have a number of advantages. First, the pressure drop as the gas flows through each tray is less. Second, the trays are easier and cheaper to fabricate and maintain.
Nevertheless, the pressure drop associated with commercial sieve trays is still significant. We have therefore set ourselves the goal of improving upon known sieve trays.
The pressure drop can be reduced by increasing the percentage open or perforate area on the surface of each tray. We have found that an attempt to increase the percentage of perforate area on each tray by changing the number and diameter of the apertures can lead to reduced point efficiency, that is the ratio between the actual separation obtained on the tray and a separation which would be obtained if thermodynamic equilibrium between gas and liquid were obtained at any point on the tray. Moreover, altering the size of the apertures does not help to counteract the tendency towards low point efficiencies at peripheral regions of the tray not lying on a straight line path between the inlet and chordal outlet of the tray. This tendency results from lower liquid flow rates over such regions of the tray.
Proposals have been made to improve the distribution of liquid on a sieve tray by imparting to the gas a component of velocity in the desired direction of the liquid flow. For example, it has been proposed to cover a conventional perforate sieve tray with a layer of expanded metal having slats which give the gas just such a component of velocity (See UK patent specification 1143772). However, this expedient does not give a total solution to the problem of a stagnant liquid and also adds to the pressure drop. Proposals have also been made to Provide a sieve tray with additional gas passages formed such that the gas is given a component of the velocity in a desired direction. (See for example U.S. Pat. No. 3,759,498). Such trays tend to have a poor turn down ratio, that is to say that they operate efficiently only within a relatively narrow range of liquid loading. Other proposals in the art include the use of vertical baffles extending well above the height of the liquid on the tray to control liquid flow. Such baffles tend to reduce the liquid-velocity and lead to the tray having poor turn down and other characteristics (see Soviet Union patent specification 1 099 973).